Identification of cells that harbor viral DNA and contribute to the HIV reservoir is a critical milestone in the search for a functional cure of HIV infected individuals. HIV replication occurs mainly in lymphoid tissues and T follicular helper cells have been identified as a major CD4+ T cell population where HIV and SIV replication take place. In addition to serving as a viral reservoir, infection of T follicular helper cells, which ae essential for germinal center (GC) formation and for antibody (Ab) production by B cells, can lead to the dysregulation of humoral immune responses, including aberrant T and B cell function, resulting in hyper-gammaglobulinemia, polyclonal activation and a decrease in antigen specific Ab production. Regulatory T cells play an important role in keeping immune responses in check. Recently, regulatory T cells that are located in GC in the murine spleen have been described and named T Follicular Regulatory cells (TFR). These cells have been found to express FoxP3, CD4, CXCR5 and Bcl6 and to develop from natural regulatory T cells in the murine thymus. Preliminary data show that cells with the phenotype of TFR (CD3+, FOXP3+, CXCR5+, BCL6+) are present in the human tonsil which leads to our hypothesis that TFR play a role in HIV pathogenesis and the maintenance of the HIV reservoir observed in T follicular helper cells. We propose to use our established methods as well as novel approaches and unique reagents to address this hypothesis with the following specific aims: 1) To determine whether T Follicular Regulatory cells (TFR) harbor HIV and to define their contribution to the HIV reservoir. 2) To determine whether TFR are functionally impaired by HIV infection, and if this contributes to the HIV-associated immune dysfunction. The present proposal represents a unique collaborative approach to elucidate cells contributing to the HIV reservoir in the lymphoid tissues, which is a major obstacle in finding a cure for HIV infected individuals. The results of the proposed research, to characterize how TFR impact B cell function in HIV infection and their role in maintaining the reservoir in lymphoid tissues, will contribute to new therapeutic approaches for a functional cure of HIV infected individuals.